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The proliferation of non-scheduled generation from renewable electrical energy sources such concentrated solar power (CSP) presents a need for enabling scheduled generation by incorporating energy storage; either via directly coupled Thermal Energy Storage (TES) or Electrical Storage Systems (ESS) distributed within the electrical network or grid. The challenges for 100% renewable energy generation are: to minimise capitalisation cost and to maximise energy dispatch capacity. The aims of this review article are twofold: to review storage technologies and to survey the most appropriate optimisation techniques to determine optimal operation and size of storage of a system to operate in the Australian National Energy Market (NEM). Storage technologies are reviewed to establish indicative characterisations of energy density, conversion efficiency, charge/discharge rates and costings. A partitioning of optimisation techniques based on methods most appropriate for various time scales is performed: from "whole of year", seasonal, monthly, weekly and daily averaging to those best suited matching the NEM bid timing of five minute dispatch bidding, averaged on the half hour as the trading settlement spot price. Finally, a selection of the most promising research directions and methods to determine the optimal operation and sizing of storage for renewables in the grid is presented. [ABSTRACT FROM AUTHOR]

One feasible solution to reduce the cost of concentrated solar power is to employ a higher temperature and efficiency supercritical carbon dioxide Brayton power cycle. An alternative heat transfer medium and thermal energy storage (TES) system therefore needs to be explored. This work considered a variety of phase change materials (PCMs) and graphite as the storage media in four indirect shell-and-tube storage configurations, including a 3-PCM and 5-PCM cascade storage, a PCM-graphite-PCM hybrid storage and a single graphite storage. The sizing and design of the TES systems were performed by using a dynamic cycling methodology based on a transient 2D numerical model. The cost of those designs were then determined by using an economic model developed inhouse. This work also investigates the impact of some geometric parameters and cost assumptions on the techno-economic performance of the TES system. The analysis suggests a scenario exists whereby a low thermal efficiency storage system which therefore utilises less tube or storage material could be more cost-effective. Overall, the cost of hybrid TES is the lowest among all studied systems, at $26.96/kWh t and $21.49/kWh t for charging temperature of 720 °C and 750 °C, respectively, followed by the 5-PCM storage at $28.06/kWh t and $21.82/kWh t , respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

A shell-and-tube design with different thermal energy storage (TES) media was investigated as a promising TES system for a next generation concentrated solar power (CSP) plant. Sensible TES using graphite, latent TES using phase change materials (PCMs) and a hybrid of both were investigated. A two-dimensional transient heat transfer model was applied to simulate the thermal performance of multiple shell-and-tube TES modules connected in series. Considering the realistic operation of the CSP plant, an intensive numerical investigation was conducted to design and size the storage system. The TES systems studied were categorized as 3-PCM cascade, 5-PCM cascade, PCM-graphite-PCM hybrid and single graphite. All the PCMs considered in this paper have been experimentally proven as potential candidate storage media. It was found that all the TES systems studied, if well designed, can meet the CSP operational requirements. In comparison, single graphite storage is less problematic, but it has the lowest storage density (47.3 kWhr/tonne). By forming a PCM-graphite-PCM sandwich configuration, the energy density was increased to 60.9 kWhr/tonne and the storage effectiveness of this hybrid system is 70.7%, the highest among all the TES systems studied. • A 2D model was validated and verified using experimental and CFD results. • Six designs of sensible, latent and sensible-latent hybrid storage were investigated. • The overall storage effectiveness of the hybrid design is the highest. • The effectiveness of the graphite system is improved by 21.2% in hybrid design. [ABSTRACT FROM AUTHOR]

Highlights • Comparison of overall exergy efficiency of different thermal energy storage systems. • Liquid sodium as HTF reduces the gap in exergy efficiency between PCM and two-tank. • High temperature sCO 2 as HTF in PCM reduces the exergy efficiency gap with two-tank. • Na with a high temperature PCM provides high storage density and exergy recovery. • Optimal temperature of PCM depends on PCM/HTF and inlet hot/cold temperatures. Abstract Different options of sensible and latent heat storage systems comprising different types of heat transfer fluids, heat storage media or phase change materials (PCMs) have been compared considering the upstream and downstream requirements of a concentrated solar power cycle. For an optimal system, analysis of both the energy efficiency and exergy recovery of the storage system and the whole cycle of heat to power is necessary. This study provides this analysis, comparing a two-tank sensible storage system to latent heat shell and tube storage system, using an analytical method, ε-NTU. For the case of molten salt as the PCM and sCO 2 as the heat transfer fluid, results show 4.02% higher exergy recovery for the two-tank system at a working temperature range 450–700 °C compared to 450–660 °C for the latent heat storage system where the upper limit is the PCM melting temperature. Compared to the Carnot cycle efficiency (the highest rate of useful energy (exergy) recovery for an isentropic process) of 70%, the overall efficiency of the two-tank system, the base case of molten salt/sCO 2 , molten salt/Na, and molten metal/sCO 2 are 57.41%, 53.39%, 55.21%, and 53.55%, respectively. The gap in efficiency between the sensible and latent heat storage systems decreases by lowering the thermal resistance of heat transfer fluid side and/or the PCM side. Specifically, the gap in efficiency decreases 45% by using liquid Na instead of sCO 2 and a 4% decrease is observed when replacing molten salt with a molten metal like aluminium as the PCM. Using PCMs with higher melting temperature is preferred with liquids with high thermal conductivity like Sodium. For a heat transfer fluid such as sCO 2 with low thermal conductivity, a higher inlet fluid temperature and/or a low temperature PCM can provide high exergy efficiency. For a specific combination of PCM and heat transfer fluid, an optimal melting temperature exists depending on the inlet temperatures during charging/discharging and the thermophysical properties of the media. The study shows that PCM storage provides opportunities to minimise causes of irreversibilities to achieve simultaneous high volumetric energy density and high exergy recovery for CSP application in comparison with the conventional two-tank sensible storage. [ABSTRACT FROM AUTHOR]

The eutectic Na 2 SO 4 -NaCl salt was investigated as a novel high temperature phase change material for solar thermal energy storage due to its low cost and melting point (626.0 °C) interested. The measured melting point and heat of fusion of the eutectic salt are 625.3 °C and 263.3 J/g, which are in agreement with their theoretical values. The eutectic salt shows good thermal stability after 100 melting-freezing thermal cycles in the range of 550–680 °C in air. Three types of the eutectic Na 2 SO 4 -NaCl-ceramic composites have been designed and evaluated to address the salt’ corrosion issues with containment materials. Results show that eutectic Na 2 SO 4 -NaCl/both α-alumina and mullite composites with a mass ratio of salt to ceramics of 1:1 (wt%) have good thermal stability without cracking as well as with weight losses of 0.50% and 0.74% and the decrease of latent heat by 1% and 2% respectively after 20 thermal cycles, where no phase separation and chemical reaction occur. Their measured melting temperature and latent heat are consistent with their theoretical values. Addition of α-alumina and mullite to the eutectic Na 2 SO 4 -NaCl salt can reduce the eutectic Na 2 SO 4 -NaCl salt’ supercooling. In contrast, the sample of the Na 2 SO 4 -NaCl/diatomite composite cracked after the cycles. [ABSTRACT FROM AUTHOR]

The mechanisms leading to corrosion in stainless steel containments for thermal energy storage through phase change materials, such as carbonates and chlorides, are crucial for understanding the degradation of these steel alloys. A comprehensive study of this area will allow for down-selection of materials suitable for solar thermal energy storage (TES) operation at an elevated temperature range. Samples of stainless steel (SS) 316L were subjected to a cyclic heat environment of 550–750 °C in air and argon headspace environments, in the presence of a carbonate salt phase change material (PCM). A series of complementary microscopy, spectroscopy and diffractometry analytical techniques were applied to the corroded SS316L. Corrosion rate, interface formation and chemical products with respect to thermal cycling are presented with associated degradation mechanism explained and comparisons are made among different gas environments and varied immersion conditions. In the PCM under ambient air conditions, steel surfaces were mainly corroded by the penetration of oxidants from air, such as H 2 O or O 2 , along grain boundaries. SS316L samples demonstrated more severe degradation when exposed to air with a small fraction of PCM vapour present compared to when they were exposed to liquid or solid PCM. Cycling in Ar resulted in less corrosion in samples as opposed to when they were exposed to an air environment. In an Ar environment corrosion is driven via the formation of chromite, while the SS316L showed a less degradation when exposed to Ar with a small fraction of PCM vapour compared to when it was immersed in PCM. The mechanisms for degradation in air and in Ar are common in that the corrosion products of Cr and Ni dissolve in the PCM thus removing any layer that could protect against corrosion. The present study provides insight into corrosion of stainless-steel when exposed to carbonate salts, in air and inert gas environments, and contributes to down-selection of materials for solar thermal energy storage. • SS316L compatibility test for PCM based solar thermal storage at high temperatures. • Determine the migration of elements including Na form the PCM into SS316L. • Identifying the depletion of steel components dissolving into the liquid PCM. • Corrosion mechanism of steels under inert gas in the presence of carbonates. [ABSTRACT FROM AUTHOR]

The intermittency of renewable energy systems remains one of the major hurdles preventing a large scale uptake of these technologies and concentrated solar power (CSP) systems are no different. However, CSP has the benefit of being able to store excess heat using thermal energy storage (TES). For the uptake of CSP with TES it must be demonstrated that the technology is both economically as well as environmentally feasible. This paper aims to investigate the economic and environmental impact of several TES options that are available for CSP systems. The investigated systems include an encapsulated phase change material (PCM) system, a coil-in-tank PCM system and a liquid sodium TES system. The economic impact in the current study refers to the capital cost (CAPEX) of each system including the tank, storage material, encapsulation cost (if applicable) and allowances for construction and engineering. The environmental impact of each system is accounted by calculating the embodied energy of each of the system components. Each storage system will be required to store a comparable amount of energy so that reliable conclusions can be drawn. The results from this analysis conclude that the encapsulated PCM (EPCM) and coil-in-tank system represent an embodied energy of roughly one third of the corresponding state-of-the-art two-tank molten salt system. Furthermore, the EPCM and coil-in-tank systems result in CAPEX reductions of 50% and 25% over the current state-of-the-art two-tank molten salt system. The liquid sodium system was found to result in higher embodied energy and CAPEX than any previously studied TES system. Finally, the advantages and disadvantages of each system was discussed and compared to previous literature. [ABSTRACT FROM AUTHOR]

A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed. [ABSTRACT FROM AUTHOR]

Demand for high temperature storage is on a high rise, particularly with the advancement of circular economy as a solution to reduce global warming effects. Thermal energy storage can be used in concentrated solar power plants, waste heat recovery and conventional power plants to improve the thermal efficiency. Latent thermal energy storage systems using phase change materials are highly thought for such applications due to their high energy density as compared to their sensible heat counterparts. This review, therefore, gives a summary of major factors that need to be assessed before an integration of the latent thermal energy system is undertaken. In addition, challenges faced when constructing and experimenting with the storage systems are mentioned. Finally, an insight on the cost analysis and the general performance metrics of the latent thermal energy storage systems is provided before conclusions are drawn. • High temperature phase change materials for CSP thermal energy storage. • Phase change materials in CSP plants. • Challenges in building LTES systems. • TES sizing and effectiveness. [ABSTRACT FROM AUTHOR]

Highlights: [•] Liquid sodium has a quite high convection heat transfer coefficient. [•] Liquid sodium delivered the highest electrical energy to grid −99.4% relative to the ideal case. [•] Gaseous HTFs achieved 87.9–91.3% of the ideal delivered electricity. [ABSTRACT FROM AUTHOR]